Metallic silicon is used for various applications depending on its purity. For example, metallic silicon having a purity of 2N is used as an alloy raw material or a semiconductor raw material. Metallic silicon having a purity of 6N or more is used for a semiconductor device, a target material, and for heat treatment. High-purity polycrystalline silicon having a purity of 10N to 11N is used as a semiconductor material or a material for solar cell application. High-purity monocrystalline silicon having a purity of 11N or more is used as a material for semiconductor device application.
Metallic silicon is lightweight and its thermal conductivity is greater than that of stainless steel so that metallic silicon may be used as an alternative to stainless steel in various devices. However, existing metallic silicon used as a semiconductor material generally has a purity of 6N or more. That purity is excessively high for a material for general industry application such as an alternative to stainless steel or quartz, and is thus less cost-effective. On the other hand, existing metallic silicon having a purity of 2N which is generally used as an alloy raw material is not suitable for use as a material for general industry application because of its poor crystallinity and difficulty in achieving reliable material strength. In particular, for example, metallic silicon with an average crystal grain diameter of less than 1 mm has insufficient material strength, and thus is not suitable for an alternative to stainless steel or quartz. Furthermore, since metallic silicon having a purity of about 2N generally has a short lifetime average value, its photoelectric conversion efficiency is low; therefore, it is unsuited for use as a material for solar cell application. Metallic silicon having a purity of 6N or more has higher photoelectric conversion efficiency; however, its purity is excessively high, and is thus less cost-effective.
A method for manufacturing high-purity metallic silicon is proposed which includes melting crude metallic silicon, and unidirectionally-solidifying and refining the molten crude metallic silicon (Patent Documents 1 and 2). However, in the case in which metallic silicon having a purity of about 3N to 5N is manufactured by melt-refining crude metallic silicon in this method, the ingot may crack easily unless the solidification rate after melting and the cooling rate after solidification are controlled appropriately. Therefore, it is impossible to obtain metallic silicon suitable for a material for general industry application. For this reason, existing metallic silicon having a purity of about 3N to 5N is generally not used as a material for general industry application.    Patent Documents 1: Japanese Patent Application, Publication No. H5-254817    Patent Documents 2: Japanese Patent Application, Publication No. H10-182135